Aromatic carbonate polymers are well-known thermoplastic materials which, due to their many advantageous properties, find use as thermoplastic engineering materials. The aromatic carbonate polymers exhibit, for example, excellent properties of toughness, impact resistance, heat resistance, and dimensional stability. Not only are molded parts prepared from aromatic carbonate polymers, but films and sheet materials as well. Noticeably lacking from their otherwise excellent property spectrum, aromatic carbonate polymers such as aromatic polycarbonate have less than desirable resistance to burning for certain applications. Thus, a wide number of flame retardant additives have been proposed for use in the aromatic polycarbonates to provide a degree of flame retardance to the finished article as necessary in any particular application.
Most often suggested to impart a mild flame retardance to the aromatic carbonate polymers is the addition of the simple charring agents such as the metal salts of aromatic sulfonic acids, specific examples of which may be found in U.S. Pat. Nos. 4,075,164 and 4,093,589. In instances where a greater degree of flame retardance is necessary, the halogenated aromatic flame retardants are employed. However, as is often the case with any additive to a thermoplastic blend, the addition of the halogenated aromatic flame retardants to the aromatic carbonate polymers produces undesirable side effects. This invention deals with a particular such side effect. The addition of the halogenated aromatic flame retardants to the aromatic carbonate polymer promotes UV instability particularly in the form of discoloration or yellowing after ultraviolet radiation exposure. This discoloration of the aromatic carbonate reduces clarity and alters appearance. Not only do transparent sheet, film, and molded articles suffer from this discoloration but likewise opaque sheet, film, and molded articles suffer as well.
Additives have been discovered which inhibit the discoloration or yellowing of flame retarded aromatic carbonate polymers after exposure to ultraviolet radiation. Thus, particularly where non-discolored aromatic carbonate polymer is desired, the utility of polycarbonate film, sheet, and molded parts is increased.